The invention relates to a two circuit braking system for vehicles, and more particularly to a master brake cylinder for such a system. The master cylinder includes an auxiliary piston within which slides a control valve disposed between a source of brake pressure and a first braking circuit. The pressure within the first braking circuit reacts back onto the control piston and also onto the control surface of a piston functioning in the main cylinder intended for the second braking circuit. A master brake cylinder of this type is described in the German Offenlegungsschrift 2 312 641.
With brake boosters of this kind the problem exists that upon the loss of the reserve pressure the travel spring excursion then represents an unnecessary waste motion which elongates the pedal travel of the brake booster upon the loss of the auxiliary energy.